Isenmühle weir in Heldenstein, DE | 2020

The solution to the undermining of Isenmühle weir in Heldenstein

The Isen River originates in Lacken, in the district of Mühldorf a. Inn. In the district of Erding, it flows through the municipalities of Isen, Lengdorf, and the city area of Dorfen. After another 76 km, the Isen River flows into the Inn River in the district of Altötting. Many streams and drainage ditches flow into the Isen River because the river plays an important role in the flood management of the region. Several weirs regulate the water inflow and outflow. Some of these Isen weirs are up to 100 years old and are therefore partly in poor condition. They need to be strengthened or replaced by appropriate measures to ensure their long-term function. The failure of a weir can lead to a high volume of water flow, which can have a significant impact on the aquatic ecology of the water-carrying vein.

Due to a construction defect, the Isenmühle weir suffered an undermining in April 2020, and the backwater behind the weir quickly dropped by 1.5 m. A week later, an old watercourse was almost dry, water plants dried up, and several larger fish were cut off from the main stream. Under enormous time pressure, a solution was developed by the engineering office AquaSoli from Traunstein to quickly restore the condition of the water before the undermining and minimize the ecological impact. The weir could no longer be strengthened and had to be demolished and replaced by a bottom ramp with a closed deck structure. The solution envisaged the immediate construction of a stone dam upstream in the Isen weir. The water could then flow back into the side arm of the Isen River, thus restoring the water level above the former weir.

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de German AMCRPS_CS-039_Germany-Wehr-Isenmuhle_DE-web.pdf
Längsschnitt durch den Rampenkörper
Wehr Isenmühle
Wehr Isenmühle
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To prevent the underflow of the existing weir structure, as well as to secure the excavation during the concrete demolition work and as a sealing element of the planned ramp structure, the steel sheet pile profile PU 22 was installed in the upper water of the existing weir system. As the weir was at risk of collapse, the sheet pile axis was moved about 12.0 m upstream of the weir sill and thus outside the fall range of the towers for the weir flap tension elements. In this position, the sheet pile served as a safeguard for the excavation and ramp head. 

Profiles without a locking seal were used to ensure seepage and thus fresh water supply to the underwater area. Despite difficult subsoil conditions and adjacent development, the sheet piles with a length of 13.0 m could be inserted into the Tertiary without any problems. The easy insertion of the sheet piles was favored by the long-standing experience of the ramming device operator and the selected sheet pile profile PU 22 with a wall thickness of 12.1 mm. 

The described measure was an emergency measure with imminent danger. The work was carried out under difficult conditions. The construction site, including transport permits and provision of sheet pile profiles, was organized within four days and started the following day, so that the water could be secured within a week. 

Only eight hours after contact, the first delivery of sheet piles for the permanent ramp structure left the manufacturer. 

The project is exemplary of a remarkable community effort by the participating companies.